This is some standard guidance that addresses the most common reasons for Flash problems in Firefox. I'm sure you've seen some of it before, but just in case:
(1) If you have any recorders/downloaders that interact with Flash media, make sure they are as up-to-date as possible, or disable them temporarily.
(2) Disable hardware graphics acceleration in Firefox and in Flash
(A) In Firefox, un-check the box here and restart:
orange Firefox button (or Tools menu) > Options > Advanced > General > "Use hardware acceleration when available"
(B) In Flash, see this support article from Adobe: http://helpx.adobe.com/flash-player/kb/video-playback-issues.html#main_Solve_video_playback_issues
(3) Disable protected mode (Windows Vista and higher)
See this support article from Adobe under the heading "Last Resort": [http://forums.adobe.com/message/4468493#TemporaryWorkaround Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?]

The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

Go to Firefox > Help > Troubleshooting Information.
Click the "Reset Firefox" button.
Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
Firefox will open with all factory defaults applied.
Further information can be found in the Reset Firefox – easily fix most problems article.

Did this fix your problems? Please report back to us!

Thank you.

Hello,
The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions, Open websites, and some Preferences.
To Reset Firefox do the following:
Go to Firefox > Help > Troubleshooting Information.
Click the "Reset Firefox" button.
Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
Firefox will open with all factory defaults applied.
Further information can be found in the Reset Firefox – easily fix most problems article.
Did this fix your problems? Please report back to us!
Thank you.